This invention relates to a hydrogen storage composition and to a method of supplying hydrogen.
Amongst the known metal hydrides, only a few exhibit properties suitable for reversible hydrogen storage. Reversibility of hydrogen sorption by a metal hydride means the capability to absorb and desorb hydrogen under practical conditions of hydrogen pressure and temperature. Most hydrides are either too stable for hydrogenation cycling so that absorption is easy but desorption of hydrogen requires very high temperatures; or too unstable so that desorption occurs readily, but absorption requires extremely high hydrogen pressure.
Stable metal hydrides include such compounds as LiH, which melts at the temperature of 680xc2x0 C. but decomposes at an even higher temperature of 720xc2x0 C., TiH2, CaH2 and ZrH2, which have to be heated up to 650xc2x0 C., 600xc2x0 C. and 880xc2x0 C., respectively, in order to release hydrogen. Re-hydrogenation of these hydrides is, however, easy and they absorb hydrogen readily even under low hydrogen pressure.
From the group of unstable metal hydrides, AlH3 and LiAlH4 are the most characteristic examples, having such a high equilibrium pressure of hydrogen that gaseous hydrogenation is in practice impossible. Only chemical reactions are effective for the formation of these unstable hydrides. On the other hand, both AlH3 and LiAlH4 desorb hydrogen readily at temperatures between 100xc2x0 C. and 130xc2x0 C. and with good kinetics.
So far, the only materials which exhibit practical, reversible properties, i.e., hydrogenation/dehydrogenation at ambient conditions, for example, LaNi5, FeTi or Tixe2x80x94V, have a hydrogen capacity of less than 2 wt. %, which is too low for practical applications.
The invention provides hydrogen storage compositions capable of reversible operation at moderately elevated temperatures of 80-180xc2x0 C., typically 100-150xc2x0 C., and having a hydrogen capacity exceeding 2 wt. %.
In accordance with one aspect of the invention there is provided a reversible hydrogen storage composition having a hydrogen capacity exceeding 2 wt. % and capable of reversible operation at temperatures of 80-180xc2x0 C. comprising i) an unstable AlH3-based complex hydride alloyed by ball milling with: ii) at least one member selected from the group consisting of: a) an element that does not readily form a hydride in a solid-state form, b) a metal which forms a stable metal hydride, c) a hydride of a metal of b), and d) an unstable AlH3 hydride complex different from said complex i), said unstable AlH3-based complex hydrides i) and ii)d) liberating hydrogen readily at temperatures of 100 to 150xc2x0 C.
In accordance with another aspect of the invention there is provided a composition of the invention in a dehydrogenated state.
In accordance with yet another aspect of the invention there is provided a method of supplying hydrogen comprising liberating hydrogen from a composition of the invention at a temperature of at least 80xc2x0 C. with formation of a dehydrogenated form of the composition, removing the liberated hydrogen, and absorbing hydrogen in the dehydrogenated form to regenerate the AlH3-based complex hydride as a future source of hydrogen.
In this specification reference to a xe2x80x9cdehydrogenated formxe2x80x9d or xe2x80x9cdehydrogenated statexe2x80x9d refers to a form or state of the hydrogen storage composition of the invention resulting from liberation of hydrogen from the composition. It is not intended to indicate that complete dehydrogenation has necessarily occurred, and contemplates both a complete dehydrogenation and a partial dehydrogenation resulting from liberation of at least part of the hydrogen content of the composition.
The reference to an AlH3-based complex hydride refers to those complex metal hydrides such as NaAlH4 or LiAlH4 which liberate hydrogen readily at moderate temperatures of the order of 100 to 150xc2x0 C., forming a dehydrogenated form or state from which the hydride form can not be regenerated with hydrogen gas, or which can only be regenerated under extreme conditions impractical in a reversible hydrogen storage composition. These AlH3-based complex hydrides are sometimes referred to as being xe2x80x9cunstablexe2x80x9d in view of their ease in liberating hydrogen and the difficulty in regeneration from the dehydrogenated form.
The invention is based on the discovery, that properties of these so-called unstable metal hydrides, which decompose easily but are very difficult to re-hydrogenate, can be altered in such a way that the required re-hydrogenation conditions are much more favourable and the hydride can be regenerated with hydrogen gas in a practical operation.
The alteration of the unstable metal hydride may be achieved by changing the chemical composition of the hydride, accompanied by mechanical grinding.
More especially the invention is concerned with hydrides based on AlH3-complex. AlH3 is very unstable and decomposes spontaneously at temperatures above 100xc2x0 C. Normally, AlH3 can not be rehydrogenated, even at extremely high hydrogen pressures, after hydrogen has been liberated from it. The same applies to other hydrides based on AlH3 complex, for example, LiAlH4, NaAlH4, Mg(AlH4)2 and Ca(AlH4)2. These hydrides offer very high hydrogen capacities, typically up to 7-wt. %, and desorb hydrogen easily at temperatures between 100xc2x0 C. and 180xc2x0 C., but normally can not be rehydrogenated at hydrogen pressures lower than 100 atmospheres.
In this invention, properties of such unstable hydrides are changed by incorporating in them other elements or hydrides, typically the incorporation may be by alloying the components together by, for example, mechanical grinding or ball milling.
A large variety of unstable AlH3-based hydrides have been investigated with different alloying components, and such alloying is found to produce dramatic change in the hydrogenation properties of the AlH3-based hydrides. As a consequence, the hydrides become reversible for practical applications because rehydrogenation can be performed at much lower hydrogen pressures.
Typical AlH3-based complex hydrides employed in the invention may be represented by formula (I):
Mx(AlH3)yHzxe2x80x83xe2x80x83(I) 
wherein M is a metal; x is an integer of 1 to 3, y is an integer of 1 or 2, and z is equal to x or 2x. Preferred examples of M are Li, Na, Be, Mg and Ca, and preferably x is 1 or 3.
Suitable AlH3-based complex hydrides for use in the invention include LiAlH4, NaAlH4, Mg(AlH4)2, Be(AlH4)2, Zr(AlH4)2, Ca(AlH4)2, Li3AlH6 and Na3AlH6 all of which change their hydrogen sorption properties when mechanically ground or ball milled in the presence of at least one member selected from the following Groups:
1. elements that do not form hydrides in a solid-state form under normal conditions, for example, metalloids such as B, C, Si, P and S, and metals such as Cr, Mn, Fe, Co, Ni, Cu, Mo, Zn, Ga, In and Sn;
2. elements which form relatively stable metal hydrides, such as Be, Mg, Ca, Ti, V, Y, Zr and La;
3. hydrides of the elements from Group 2 above such as BeH2, MgH2, CaH2, TiH2, VH2, YH2, ZrH2 and LaH2;
4. other hydrides based on the AlH3-complex.
These additions, alone or in mixtures, are able to change the sorption properties of the AlH3-based complex hydrides. The mechanism of the change is not fully understood, but it is probable that different mechanisms are involved with the different classes of additive.
The probable mechanisms of altering hydrogenation properties of AlH3based hydrides are as follows:
i) interstitial alloying of the AlH3-based hydride.
This mechanism is most probable in the case of metalloids as, for example, boron and carbon.
ii) substitutional alloying accompanied by catalysis.
This mechanism is expected to apply to most metal additions from group 2 elements.
iii) synergetic effect of hydrogen sorption in mixtures of hydrides.
This mechanism was found in mixtures of AlH3-based hydrides with hydrides of groups 3 and 4 above and also as a result of ball milling with elements from group 2 above. In the latter case, however, formation of the respective hydrides, listed in group 3, can occur during hydrogenation/dehydrogenation cycling of the main AlH3-based hydride. Depending on the addition, there are essentially two kinds of behaviour of hydride mixtures. One is of a kinetic character, when the basic hydride does not react with the addition. In this case the addition acts as a hydrogen carrier or catalyst and improves the reaction kinetics. The second mechanism is based on formation of new complex hydrides or more complicated hydride complexes. In this case thermodynamic properties of the main AlH3-based hydride are significantly altered, resulting in changed equilibrium pressures for hydride formation.
The above mechanisms for the improvement of hydrogen sorption properties of AlH3-based complex hydrides, as a result of changes in the chemical compositions by means of mechanical alloying with additions, were studied in various AlH3-based complex hydrides with a number of additions from the above groups of materials. Within one family of additions, the amounts of additions were varied.
Typically the molar ratio of the AlH3-based complex hydride to the addition was changed in the range between 10:1 to 1:3. Samples of AlH3-based complex hydrides with no additions at all, but ball milled at the same conditions as the samples with additions, were also studied. As a general conclusion it was found that in each case ball milling alone improved kinetic properties of the AlH3-based complex hydride, but ball milling with additions improved them much more remarkably and evidently could change thermodynamical properties of the main hydride.
Although the detailed nature of these changes has not been fully determined, some general conclusions can be described as follows, in connection with the mechanisms proposed above.
Interstitial character of alloying with metalloids is confirmed by x-ray diffraction analysis. For example, addition of C to NaAlH4 in the molar proportion of 1:1 does not change the x-ray diffraction pattern of NaAlH4 and no other reflections were observed which could indicate formation of other phases. Also, no reflections or halos from crystalline or amorphous carbon can be seen in the x-ray diffraction pattern. At the same time, however, this material exhibits hydrogenation properties which differ dramatically from conventional NaAlH4. As reported previously [1,2] NaAlH4 has such a high equilibrium pressure of hydrogenation that it was normally impossible to rehydrogenate it after decomposition. Only recently Bogdanovic discovered a catalyst that enabled rehydrogenation of NaAlH4 [3, 4]. However, a very high hydrogen pressure of 150 atm was still necessary to perform absorption at 170xc2x0 C.
A material according to the present invention, being a ball milled mixture of NaAlH4 and C exhibits reversible hydrogen sorption properties at much lower pressures and with much faster kinetics. Equilibrium pressure for this material is, for example, about two times lower at 140xc2x0 C., than the reported values for conventional NaAlH4 [1, 2]. This means that much lower hydrogen pressures are required to effectively perform hydrogen absorption. Moreover, kinetics of the hydrogenation/-dehydrogenation cycles remarkably exceeds the reaction rates observed not only for the conventional NaAlH4, but also for the catalysed NaAlH4 of the prior art [3, 4]. For example, the catalysed NaAlH4 desorbs 2 wt. % of hydrogen at 160xc2x0 C. within about 6 hrs. [3, 4], while NaAlH4 with C of the invention can desorb the same amount of hydrogen within only 30 min. For comparison, conventional NaAlH4 without catalyst requires more than 50 hrs. at 160xc2x0 C. to desorb 2 wt. % of hydrogen.
In another example boron is found to be very effective in changing the hydrogen sorption properties of different AlH3-based complex hydrides. In complex hydrides boron shifts the equilibrium pressure of hydrogen towards lower pressures, i.e., stabilizes the AlH3-based complex hydride. This is very advantageous because as a result these hydrides can effectively operate at lower hydrogen pressures.
Addition of silicon also results in significantly enhanced kinetics of hydrogenation cycling of the AlH3-based complex hydrides.
Additions of metals, for example, Cu, Ni, Fe and Zn also improve sorption properties of AlH3-hydride complexes. The presence of Cu, Fe or Mn can be seen in the x-ray diffraction pattern, but with a clearly reduced size of the metal grains, which is very advantageous from the point of view of the possible catalytic action of the additions.
AlH3-based complex hydrides ball milled with elements which easily form hydrides, from group 2 above, are among the most interesting materials because of the variety of the possible combinations of the hydride mixtures. The additions can be introduced into the mixture in the form of elements or their hydrides, according to groups 2 and 3 above. Ball milling with these additions results in the formation of hydride complexes with changed thermodynamical properties. For example, ball milling of NaAlH4 with zirconium or with its hydride results in such a change of the equilibrium pressure that effective absorption of hydrogen can be performed at 60 to 80 atm instead of 150 atm., as reported previously for catalysed NaAlH4 [3, 4]. Kinetics of hydrogenation cycling are also many times faster at similar temperatures than catalysed NaAlH4. Excellent hydrogen sorption properties appear to be even more enhanced when the mixtures are in the nanocrystalline form, with the components being extremely finely intermixed.
Suitably the AlH3-based complex hydride and the additive from one or more of groups 1, 2, 3 and 4 above, have a particle size below 100 xcexcm, preferably below 50 xcexcm.
The hydrogen storage compositions of the invention liberate hydrogen at a temperature of at least 80xc2x0 C., generally 80 to 180xc2x0 C. and typically 100 to 180xc2x0 C.
Hydrogen absorption into the dehydrogenated form of the hydrogen storage composition of the invention is suitably carried out at a temperature of 80 to 150xc2x0 C. typically 100 to 150xc2x0 C., and a hydrogen pressure of 20 to 100, preferably 30 to 80 atm for a period of 0.25 to 5 hours, preferably 0.5 to 3 hours.
The hydrogen storage compositions of the invention have a hydrogen capacity of 2 to 7, more usually 3 to 7 wt. %.